Current biased dual DBR grating semiconductor laser

ABSTRACT

A laser heterostructure having an active layer, a lateral waveguide terminating in an output aperture, and a gain section with a current drive electrode. A rear surface distributed Bragg grating with a tuning current electrode is formed on a surface of said laser heterostructure. The laser also includes a front surface distributed Bragg grating with a tuning current electrode on a surface of the laser heterostructure. The front surface distributed Bragg grating is closer to the output aperture than the rear surface distributed Bragg grating, There is a space between the rear surface distributed Bragg grating and the front surface distributed Bragg grating. A current drive electrode is formed on the space. Operation is best when the front surface distributed Bragg grating has adequate reflectivity at the Bragg wavelength with minimal scattering loss at other wavelengths, particularly at the wavelength of the rear surface Bragg grating.

STATEMENT OF GOVERNMENT INTEREST

This invention was made with government assistance under NationalScience Foundation grant ECS 99-00258 and the Air Force Office ofScientific Research contract F49620-96-1-0163 COL. The Government hascertain rights in this invention.

FIELD OF THE INVENTION

The field of the invention is semi conductor lasers.

BACKGROUND OF THE INVENTION

Multiwavelength optical sources are important components in applicationssuch as wavelength division multiplexing, optical remote sensing, andoptical data processing. Multiple wavelengths are commonly achieved froma single output by integrating the output from multiple, discretelasers. This can lead to large and complex chip design, however. Otherapproaches, such as cascaded strongly gain-coupled DFB (distributedfeedback) lasers, rely on the reflectivity comb from integrated multiwavelength feedback mechanisms for their operation. Such integrateddesign of the multi wavelength feedback makes it difficult to select andtune a wavelength while leaving other wavelength(s) unaffected.

Thus, there is a need for an improved semiconductor laser capable ofdual wavelength operation. It is an object of the invention to providesuch a laser.

SUMMARY OF THE INVENTION

These needs are met by the present a laser heterostructure having anactive layer, a lateral waveguide terminating in an output aperture, anda gain section with a current drive electrode. A rear surfacedistributed Bragg grating with a tuning current electrode is formed on asurface of said laser heterostructure. The laser also includes a frontsurface distributed Bragg grating with a tuning current electrode on asurface of the laser heterostructure. The front surface distributedBragg grating is closer to the output aperture than the rear surfacedistributed Bragg grating, There is a space between the rear surfacedistributed Bragg grating and the front surface distributed Bragggrating. A current drive electrode is formed on the space. Operation isbest when the front surface distributed Bragg grating has adequatereflectivity at the Bragg wavelength with minimal scattering loss atother wavelengths, particularly at the wavelength of the rear surfaceBragg grating.

In the present laser, dual-wavelength operation is easily achieved bybiasing the gain section. A relatively low coupling coefficient, κ, inthe front grating reduces the added cavity loss for the back gratingmode. Therefore, the back grating mode reaches threshold easily. Thespace section lowers the current induced thermal interaction between thetwo uniform grating sections, significantly reducing the inadvertentwavelength drift. As a result, a tunable mode pair separations (Δλ) assmall as 0.3 nm and as large as 6.9 nm can be achieved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a preferred RW-DBR laserheterostructure of the invention;

FIG. 2 illustrates threshold currents for three prototype devices of theinvention measured at different temperatures ranging from 20° C. to 60°C.;

FIG. 3 illustrates longitudinal mode spectra of a prototype devicemeasured at different tuning currents from 0 mA to about 35 MA with thelaser current fixed at 50 MA at 60° C.; and

FIG. 4 shows peak wavelengths of the prototype laser measured in FIG. 3.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to FIG. 1, a preferred ridge waveguide-distributed Braggreflector (RW-DBR) laser heterostructure 10 of the invention is shown.The laser heterostructure emits from an output aperture 11. A rearsurface distributed Bragg grating 12 is formed on a surface of saidlaser heterostructure. The laser also includes a front surfacedistributed Bragg grating 14 on a surface of said laser heterostructure.The front surface distributed Bragg grating 14 is closer to the outputaperture 11 than the rear surface distributed Bragg grating 12. There isa space 15 between the rear surface distributed Bragg grating 12 and thefront surface distributed Bragg grating 14. A tuning electrode 16 isformed on the front surface distributed Bragg grating 14, and the regionbetween the front distributed Bragg grating and the output aperture 11forms a gain section 17. A current drive electrode 18 is formed on thespace 15, and a tuning electrode 19 on the rear surface distributedBragg grating. An additional current drive electrode 20 is formed on thegain section 17. Operation is best when the front surface distributedBragg reflector 14 has an optimal design such that adequate reflectivityat the Bragg wavelength is obtained with minimal scattering loss atother wavelengths, particularly that of the rear surface distributedBragg grating 12. The coupling coefficient of the rear surfacedistributed Bragg grating 12 is less critical and may be higher thanthat of the front surface distributed Bragg grating 14. Prototypes havebeen formed and tested. Their operation illustrates some advantages ofthe invention, but the particular prototype parameters, i.e. gratingdimensions, separation lengths, etc., do not limit the broader aspectsof the overall structure which forms part of the invention as shown inFIG. 1. The prototypes and their operation will now be discussed.

In prototypes which were tested, uniform front and back gratings with anidentical period were used, and current injection into the tuningelectrode 16 defined the wavelength separation between the mode pairs.The laser operates in a single-wavelength mode with no current appliedto the tuning electrode 16, and dual-wavelength operation is achievedwhen current is applied to the tuning electrode 16. The prototypeInGaAs-GaAs asymmetric cladding RW-DBR lasers operate in adual-wavelength mode with tunable mode pair separations as small as 0.3nm and as large as 6.9 nm.

Epitaxial layers for the asymmetric cladding separate confinementheterostructure (SCH) prototypes were grown by atmospheric pressuremetalorganic chemical vapor deposition (MOCVD) in a vertical reactor ona (100) GaAs:n⁺ substrate. After the growth, silicon dioxide wasdeposited by plasma-enhanced chemical vapor deposition. Grating periodswere defined by electron beam lithography. The gratings were transferredinto the silicon dioxide using Freon 23 reactive ion etching (RIE).Plasma-Therm SLR inductively-coupled plasma (ICP) RIE was used to etchthe gratings into the epitaxial layers with SiCl₄. A 4 μm lateral ridgewas defined using standard photolithography and etched in a sulfuricacid solution. A liftoff metallization was performed to provide separatecontacts for the laser and the tuning elements. No coatings are appliedto the facets. The prototype DBRs utilize uniform gratings with the sameBragg period, but gratings having different periods will also workwithin the above described limitations of reflectivity and scatteringlosses. The metal liftoff was aligned over the gratings to provideelectrical isolation for the common gain section and the two DBRsections. The two DBR sections are physically separated in order toreduce the inadvertent heating mentioned.

Three different device dimensions were investigated. Table I is asummary of physical dimensions of these devices. The ridge heights andridge widths for all three devices are 0.15 μm and 3.5 μm, respectively.All prototype DBRs had a uniform grating with a period of 166.0 nm.

TABLE I Device (a) (b) (c) L_(gain) 500 525 550 L_(front DBR) 100 75 50L_(space) 100 75 50 L_(back DBR) 100 100 100

Prototype testing was performed continuous wave (CW) with a heat sinktemperature maintained by a thermoelectric (TE) cooler. Only the gainsection is biased to achieve lasing. FIG. 2 shows threshold currents forthe three different devices in Table I measured at differenttemperatures. At 20° C., the threshold currents for devices (a), (b),and (c) are 12 mA, 28 mA, and 32 mA, respectively, and the uniformgrating period of 166.0 nm results in a single nominal lasing wavelength(λ_(front) of ˜1077.4 nm. As expected, device (a), with the shorter gainsection and the longer front DBR section (i.e. stronger reflectivity),exhibits the lowest threshold current for all temperatures. FIG. 2 alsoshows that the threshold currents decrease for all three devices as theTE cooler temperature is increased from 20° C. to 60° C. Thresholdcurrents for devices (a), (b), and (c) are as low as 7.1 mA, 18 mA, and18.5 mA, respectively. This trend can be accounted for by the Braggcondition being red-shifted with respect to the gain peak. As thetemperature of the device increases, the gain peak tunes toward theBragg condition, leading to a lower threshold current. As expected, nolasing is observed when only the tuning pad is biased.

Dual-wavelength operation occurs in devices (b) and (c) when the lasersare biased high enough to support two lasing modes. Both devices exhibitsimilar trends in their performance. No additional current is requiredfor dual-wavelength operation. The coupling coefficient, κ, of the frontDBR is estimated to be ˜80 cm⁻¹. This lower value of κ is believed to bean important feature that contributes to the dual-wavelength operationof these devices. For these devices with deeply etched surface gratings,the higher threshold condition for the second lasing mode for the backDBR (λ_(back)) results from the added cavity loss caused by scatteringin the front DBR. By reducing the loss caused by the front gratingsection, λ_(back) reaches threshold more easily. While device (a)exhibits the lowest threshold current, it does not exhibitdual-wavelength operation: Because the added loss for λ_(back) in device(a) with the longest front grating (L_(f)) and the spacing (L_(s))sections, λ_(back) does not reach threshold. Therefore, a low κL productfor the front grating is an important factor determining the thresholdcondition of λ_(back). However, κ for the front grating still needs tobe sufficiently high in order for the front grating to act as a goodwavelength-selective reflector. Minimizing cavity loss for the backgrating section should be carefully balanced with sufficient reflectionfor λ_(back).

Once dual-wavelength operation is achieved by biasing the gain section,injection of current into the front DBR section results in wavelengthtuning. FIG. 3 shows the longitudinal mode spectra of device (c)measured at different tuning currents from 0 mA to 35 mA with the lasercurrent fixed at 50 mA at 60° C. As noted earlier, the gain peak of thematerial overlaps better with the Bragg condition at elevated operatingtemperatures, facilitating dual-wavelength operation and therefore,tuning. Because of thermal coupling between the gain section and thefront DBR, the wavelength of λ_(front) is slightly longer than λ_(back).FIG. 4 shows the peak wavelengths the RW-DBR laser presented in FIG. 3.Closed circles represent the front DBR mode (λ_(front)), and opencircles represent the back DBR mode (λ_(back)). As the tuning currentincreases, both modes shift towards longer wavelengths, with the tunablewavelength shifting further. When the tuning current is increased from 0mA to 35 mA, λ_(front) increases by 7.4 nm, tuning from 1081.6 nm to1089.0 nm. Therefore, the RW-DBR laser exhibits a tunable mode pairseparation (Δλ) as small as 0.3 nm and as large as 6.9 nm. At highercurrents λ_(front) jumps a lower wavelength that is more favorable forlasing. Device (b) also exhibits similar trends. FIG. 4 also illustratesthe effect of inserting a 50-μm spacing section between the to reducethe inadvertent drifting of λ_(back). When the tuning current isincreased from 0 mA to 40 mA, the wavelength of λ_(back) increases byonly 0.9 nm, from 1081.3 nm to 1082.2 nm (0.025 nm/mA). For device (b),which has a longer spacing section (75 μm), the drift in λ_(back) isfurther reduced to ˜0.015 nm/mA.

Thus, by simply biasing the gain section, a laser of the inventionoperates in dual-wavelength. Lower coupling coefficient, κ, in the frontgrating reduces the added cavity loss for the back DBR mode λ_(back),and therefore, λ_(back) reaches threshold more easily. Also, theaddition of a spacing section reduces the current induced thermalinteraction between the two uniform grating sections, significantlyreducing the inadvertent wavelength drift. As a result, a tunable modepair separations (Δλ) as small as 0.3 nm and as large as 6.9 nm can beachieved.

While various embodiments of the present invention have been shown anddescribed, it should be understood that other modifications,substitutions and alternatives are apparent to one of ordinary skill inthe art. Such modifications, substitutions and alternatives can be madewithout departing from the spirit and scope of the invention, whichshould be determined from the appended claims.

Various features of the invention are set forth in the appended claims.

What is claimed is:
 1. A semiconductor laser, the laser comprising: alaser heterostructure having an active layer, a lateral waveguideterminating in an output aperture, and a gain section; and a rearsurface distributed Bragg grating on a surface of said laserheterostructure; a first tuning electrode formed on said rear surfacedistributed Bragg grating; a front surface distributed Bragg grating ona surface of said laser heterostucture and closer to said outputaperture than said rear surface distributed Bragg grating, the gainsection being between said front surface distributed Bragg grating andsaid output aperture; a second tuning electrode formed on said frontsurface distributed Bragg grating; a space between said rear surfacedistributed Bragg grating and said front surface distributed Bragggrating; a first current drive electrode formed on said gain section; asecond current drive electrode formed on said space.
 2. The laseraccording to claim 1, wherein said front surface distributed Bragggrating has a period simultaneously providing adequate reflectivity at aBragg wavelength of said front surface distributed Bragg grating andminimizing scattering loss at a wavelength of said rear surfacedistributed Bragg grating.
 3. The laser according to claim 2, whereinsaid rear surface distributed Bragg grating and said front surfacedistributed Bragg grating have a common Bragg period.
 4. The laseraccording to claim 2, wherein said laser heterostructure comprisesInGaAs-GaAs in a ridge waveguide distributed Bragg reflector formation.5. The laser according to claim 2, wherein the coupling coefficient ofsaid front distributed Bragg gratings is approximately 80 cm⁻¹.
 6. Thelaser according to claim 1, wherein a coupling coefficient of a frontone of said pair of surface distributed Bragg grating is sufficientlyhigh to provide wavelength selective reflectivity and sufficiently lowto reduce cavity loss for a rear one of said pair of surface distributedBragg gratings.